Generally, a single spiral propeller is used in a propulsion device for a ship. However, the propulsion device having a single propeller may not acquire propulsive force from rotational energy of water streams, and thus causes substantial energy loss.
A counter rotating propeller (CRP) type propulsion device may acquire propulsive force from the rotational energy lost. In the counter rotating propeller type propulsion device, two propellers installed on the same axis generate propulsive force via counter rotation thereof. A rear propeller acquires propulsive force from rotational energy of fluid passing through a front propeller. Accordingly, the counter rotating propeller type propulsion device may exhibit higher propulsion performance than the propulsion device having the single propeller.
However, the counter rotating propeller type propulsion device includes a counter rotation unit in which the two propellers are reversely rotated with respect to each other, a hollow shaft or the like, and thus it is relatively difficult to manufacture and install the counter rotating propeller type propulsion device, and also a high technical standard is required to stably operate the propulsion device while maintaining reliability thereof.
Further, in the counter rotating propeller type propulsion device, a separate unit for emergency operation which may operate a ship with a remaining propeller, when one of the propellers may not be driven by a breakdown in the counter rotation unit, is required.